1. Technical Field
The present invention relates to fluid dynamic bearings, motors, and recording-medium driving apparatuses.
2. Background Art
Fluid dynamic bearings are used as shaft bearings for apparatuses requiring high rotational accuracy. This type of bearing has a gap, filled with a working fluid, between a shaft and a housing to rotatably support the shaft relative to the housing. When the bearing rotates, the working fluid produces a dynamic pressure to keep the shaft and the housing out of contact with each other.
An example of an apparatus requiring high rotational accuracy is a recording medium such as a hard disk. The rotational speed and accuracy of hard disks are becoming increasingly higher with increasing recording density.
Fluid dynamic bearings are suitable for use in spindle motors for driving recording media having increasingly higher rotational speed and accuracy, including hard disks.
The demand for reduction in the size and thickness of recording media such as hard disks, as well as higher rotational speed and accuracy, has been increasing in recent years. Accordingly, various techniques have been disclosed to provide thinner fluid dynamic bearings (for example, see Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2001-65552 (for example, pages 3 and 4 and FIG. 1).
A thrust bearing of a dynamic bearing unit disclosed in Patent Document 1 above includes a sleeve having a flange (thrust plate) defining a thrust dynamic bearing portion and a shaft member having a circumferential portion surrounding the flange. A fluid such as oil is provided between axially opposing surfaces of the flange of the sleeve and the circumferential portion of the shaft member to produce a dynamic pressure when the shaft member rotates. The flange has pressure-generating grooves for producing dynamic pressure in the oil when the shaft member rotates.
The dynamic bearing unit also includes an absorbing sheet for absorbing oil leaking from the thrust bearing.
If, however, the dynamic bearing unit is started with the surface of the flange where the pressure-generating grooves are provided and the inner surface of the shaft member close to each other, the grooves attract the surrounding oil to produce a negative pressure that causes bubbles in oil-poor regions. These bubbles degrade the lubrication performance of the oil and can thus cause problems such as vibration due to unstable rotation and, at worst, fusion of the bearing due to metal-to-metal contact.
In addition, the oil can leak out of the thrust bearing because the bubbles squeeze the oil. Although the absorbing sheet absorbs the leaking oil, an oil film forms between the thrust bearing and the absorbing sheet. This oil film is no longer easy to cut off and is thus constantly fed to the absorbing sheet. Accordingly, the absorbing sheet continues to absorb the oil until becoming saturated. As a result, the amount of oil provided on the thrust bearing decreases and, furthermore, the oil can be totally depleted. This makes it difficult to maintain the operation of the fluid dynamic bearing.